LIMNOLOGICAL OBSERVATIONS IN THE UPPER MISSISSIPPI. 
419 
When considering the data obtained by this method one has to remember that 
the figures representing the amount of plankton do not represent the total quantity 
of organisms living in the water, although they may be used in a comparative study 
of the productive capacities of various lakes. But a great mistake will be made if, 
on the basis of these data, one attempts to compute the absolute quantities of the 
living material in the inland waters. 
One of the purposes of the present investigation of the upper Mississippi River 
was to find out how the life in the river has been affected by the construction of the 
Keokuk Dam. It is shown above that there is an increase of plankton, especially in 
the copepod and cladoceran population, in the newly formed Keokuk Lake. This 
increase in plankton production occurs only at low stages of water and disappears 
during the rise of the river. It means that from a biological point of view the differ- 
ence between the river and lake exists only at a low stage and can disappear at every 
sudden rise of water. These conditions are probably peculiar to all river lakes, and 
of course they are of great importsfince to the organic life in the lakes. 
The productive capacity of such river lakes as Lake Keokuk is lessened by the 
instability of the hydrographic conditions. Nevertheless, the increase of plankton 
in Lake Keokuk during low stages of water indicates the increase of its general 
productive capacity. Therefore it would be very interesting to know if there can 
be found any indications of the increase of fish resources in this lake since the dam 
was built. Such information is found in the analysis of the statistics of the com- 
mercial fisheries in Lake Keokuk and in Lake Pepin made by R. E. Coker and 
E. S. Stringham '(1921). The authors, having analyzed the statistical data of 
fisheries in 1914 and 1917, came to the conclusion that the total catch of fish in 
Lake Pepin in 1917 was 60 per cent greater than in 1914, whereas the catch in 
Lake Keokuk had increased 172 per cent. It must be born in mind, however, 
that according to the authors “between the years 1914 and 1917 the prices of fishery 
products had risen substantially and doubtless proved a stimulus to the fisheries; 
but it does not appear at all probable that the stimulation due to price could have 
had so pronounced an effect as to create an appearance of abundance where actual 
scarcity prevailed.” 
It is interesting to note that the total increase in catch of fish in Lake Keokuk 
was nearly three times greater than the increase in Lake Pepin. As Keokuk Dam 
was completed in 1913 this appears to be due to the increased productive capacity 
of the lake. The following fishes in Lake Keokuk show substantial increases: 
Buffalofish, catfish, fresh-water drum, and German carp. Three show a decrease: 
Eels, sturgeon, and suckers. The buffalofish is the most valuable commercial 
fish in both lakes, and the catch of this fish in Lake Keokuk increased from 249,900 
pounds in 1914 to 696,543 pounds in 1917. It appears that the new conditions in a 
recently formed lake have afforded the opportunity for a substantial development 
of buffalofish as a natural resource. The commercial statistics refer, of course, to a 
limited number of fishes, but the statistics of the capture of game fish (black bass, 
crappie, pike, and suckers) also suggest an increased abundance of these fishes. 
The deductions from the fisheries statistics agree with the results obtained 
from the present investigation. The body of water held in check by the Keokuk 
Dam obviously affords more favorable conditions for the development of organic 
